This invention relates to anodizing aluminum. More particularly, this invention relates to improvements in the anodizing to produce highly reflective surfaces on bright rolled aluminum materials which have not been subjected to a preliminary brightening process prior to anodization. 2. Description of the Prior Art
Highly reflective surfaces may be produced on an aluminum material by proper selection of the alloy constituents, bright rolling or mechanical polishing of the aluminum surface and processing of the highly polished or bright rolled surface in a brightening bath which may comprise either electrobrightening or chemical brightening. The highly reflective surface so produced is then protected by anodizing the aluminum to provide a thin, transparent, protective layer of aluminum oxide on the surface as is well known to those skilled in the art. Various attempts at improving the reflectivity of the product have been proposed through the years. One approach is to vary the type of brightener used to treat the aluminum surface prior to anodizing. Typical of such an approach is the aluminum phosphate chemical brightening bath disclosed in U.S. Pat. No. 3,530,048 which uses a combination of aluminum phosphate, nitric acid, phosphoric acid, and copper sulfate. The brightened aluminum surface, according to the patentees, is then anodized in a sulfuric acid bath having a concentration of from 12 to 20 wt. % at a temperature of 20.degree. to 26.6.degree. C. (70.degree. to 80.degree. F.) using a current of about 9.26 to 13.89 amperes per square decimeter (10 to 15 amperes per square foot).
It is also known to vary the alloy constituents to improve the reflectivity of the aluminum surface. U.S. Pat. No. 3,720,508 discloses an aluminum alloy used in the production of a highly reflective aluminum surface which contains from 0.5 to 3% magnesium, from 0.2 to 0.5% silver, from 0.001 to 0.2% iron and from 0.001 to 0.15% silicon.
It is also known to provide additives in the anodizing bath to attempt to improve the bright or reflective surface of aluminum. For example, U.S. Pat. No., 3,671,333 provides for the addition of a natural or synthetic hydrophilic colloid to the reflective aluminum surface during anodizing of the aluminum by adding the colloid to the anodizing bath. Surface coatings produced during the anodization are alleged to be much thinner and apparently more compact than previous anodized aluminum coatings which the patentees allege is believed to be due to the larger molecule of the colloid forming as a colloidate on the reflective surface which apparently compacts the aluminum oxide formed. The thinner coating is then alleged to provide better reflectivity while eliminating the disadvantages of a thin normal anodized coating.
Other attempts at varying the anodization process include the use of AC anodizing using a sulfuric acid bath as shown in British Pat. No. 1,439,933. High current densities of 1 to 10 amperes per square decimeter (about 10 to 90 amperes per square foot) are proposed in U.S. Pat. No. 4,252,620 for use with a highly concentrated sulfuric acid anodizing bath containing 50 to 60% sulfuric and oxalic acid or nickel sulfate to produce a porcelain-like texture although no improvement in reflectance is alleged or apparently desired by the patentee.
U.S. Pat. No. 4,225,399 discloses an anodizing process using a current density greater than 1.5 Amps per square decimeter (13.89 Amps per square foot) in a 20-30 wt. % sulfuric acid anodizing bath at either 30.degree. C..+-.2.degree. C. for soft oxide films or 5.degree. C..+-.2.degree. C. for hard anodizing coatings. Again, the patentee makes no mention of such anodizing procedures having any effect on the reflectance of the resultant product.
It is, therefore, apparent that heretofore little, if any, attempts have been made to improve the reflectivity of an aluminum alloy by altering the anodization parameters to maximize the total reflectance of the anodized aluminum surface of the aluminum material.
In our aforementioned parent patent, U.S. Pat. No. 4,483,750, cross-reference to which is hereby made, we disclosed and claimed a novel anodizing process which produced a surprisingly superior reflectance. In the process claimed therein, the aluminum surface is first subjected to a brightening process and then is anodized by the claimed process. Subsequently, however, we discovered, as described and claimed in our aforementioned parent application Ser. No. 651,912, that the process was so effective, it could be applied to an aluminum surface which had not been previously subjected to a brightening process and still yield an anodized product having a reflectance which, while not quite as good as the results described in our aforementioned parent patent, were still as good or better than prior art reflectances obtained using prior art processes on surfaces which had been previously subjected to brightening processes prior to anodization.